DESCRIPTION

The
kill() system call
can be used to send any signal to any process group or process.

If pid is positive, then signal sig is sent to the
process with the ID specified by pid.

If pid equals 0, then sig is sent to every process in the
process group of the calling process.

If pid equals -1, then sig is sent to every process
for which the calling process has permission to send signals,
except for process 1 (init), but see below.

If pid is less than -1, then sig is sent to every process
in the process group whose ID is -pid.

If sig is 0, then no signal is sent, but error checking is still
performed;
this can be used to check for the existence of a process ID or
process group ID.

For a process to have permission to send a signal
it must either be privileged (under Linux: have the
CAP_KILL capability), or the real or effective
user ID of the sending process must equal the real or
saved set-user-ID of the target process.
In the case of
SIGCONT it suffices when the sending and receiving
processes belong to the same session.

RETURN VALUE

On success (at least one signal was sent), zero is returned.
On error, -1 is returned, and
errno is set appropriately.

ERRORS

EINVAL

An invalid signal was specified.

EPERM

The process does not have permission to send the signal
to any of the target processes.

ESRCH

The pid or process group does not exist.
Note that an existing process might be a zombie,
a process which already committed termination, but
has not yet been
wait(2)ed
for.

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001.

NOTES

The only signals that can be sent to process ID 1, the
init process, are those for which
init has explicitly installed signal handlers.
This is done to assure the
system is not brought down accidentally.

POSIX.1-2001 requires that kill(-1,sig) send sig
to all processes that the calling process may send signals to,
except possibly for some implementation-defined system processes.
Linux allows a process to signal itself, but on Linux the call
kill(-1,sig) does not signal the calling process.

POSIX.1-2001 requires that if a process sends a signal to itself,
and the sending thread does not have the signal blocked,
and no other thread
has it unblocked or is waiting for it in
sigwait(3),
at least one
unblocked signal must be delivered to the sending thread before the
kill() returns.

Linux Notes

Across different kernel versions, Linux has enforced different rules
for the permissions required for an unprivileged process
to send a signal to another process.
In kernels 1.0 to 1.2.2, a signal could be sent if the
effective user ID of the sender matched that of the receiver,
or the real user ID of the sender matched that of the receiver.
From kernel 1.2.3 until 1.3.77, a signal could be sent if the
effective user ID of the sender matched either the real or effective
user ID of the receiver.
The current rules, which conform to POSIX.1-2001, were adopted
in kernel 1.3.78.

BUGS

In 2.6 kernels up to and including 2.6.7,
there was a bug that meant that when sending signals to a process group,
kill() failed with the error
EPERM if the caller did have permission to send the signal to any (rather
than all) of the members of the process group.
Notwithstanding this error return, the signal was still delivered
to all of the processes for which the caller had permission to signal.